The present invention relates generally to the construction of a filter press membrane electrolytic cell for the production of chlorine, alkali metal hydroxides or other caustics and hydrogen, wherein each electrolytic cell unit has at least one central electrode assembly sandwiched between at least two end electrode assemblies to form a closed system for the efficient utilization of the materials circulated therethrough. More particularly, the present invention relates to an improved electrolyte recirculation system wherein restrictor apparatus is utilized in the feed line to each electrode to selectively control the recirculation rate of the electrolyte to thereby control the level of electrolyte/gas foaming that occurs in the disengager.
As products of the electrolytic process, chlorine and caustic have become large volume commodities as basic chemicals which are an integral part of Western civilization as it is known today. The overwhelming amounts of these chemicals are produced electrolytically from aqueous solutions of alkali metal chlorides. Cells which have traditionally produced these chemicals have come to be known commonly as chlor-alkali cells. The chlor-alkalicells generally were of two principal types, the deposited asbestos diaphragm-type electrolytic cell of the flowing mercury cathode type. Comparatively recent technological advances, such as the development of the dimensionally stable anode and various coating compositions, have permitted the gaps between the electrodes to be substantially decreased and thereby dramatically increased the energy efficiency in the operation of these energy-intensive units. The development of a hydraulically impermeable membrane has promoted the advent of filter press membrane electrolytic cells which produce a relatively uncontaminated caustic product, obviating the need for caustic purification and concentration processing steps. The use of a hydraulically impermeable planar membrane has been most common in bipolar filter press membrane electrolytic cells. However, advances continue to be made in the development of monopolar filter press membrane cells.
Gas separators or disengagers have been utilized, especially in monopolar filter press membrane cells, to permit the chlorine gas to separate from the anolyte fluid during the electrolytic process. The anolyte disengager typically includes a layer of liquid anolyte along its bottom portion, a layer of foam within which various gases such as O.sub.2, CO.sub.2 and chlorine are present, and the separated chlorine and other gases in the top layer. Naturally, in a process designed to produce chlorine gas, efficiency of the apparatus is gauged by its ability to have the chlorine gas separate or rise up through and out of the anolyte fluid. It has been determined in testing that excessive amounts of foam in the anolyte disengager can cause carryover of foam into the gas flow lines leading to undesirable pressure surges during operation, while too little foam in the disengager may indicate that excessive chlorine gas separation is taking place within the anode chamber which may be damaging to the membranes because of the high concentration of chlorine gas within the anode and detrimental to the energy efficiency of the cell.
To control the production of gas during operation, electrolyte is circulated through a cell between the electrodes and the disengagers. It has been found that the greater the rate of recirculation of electrolyte, the greater is the amount of foam that is formed within the anolyte disengager. A similar relationship has been found to exist in the catholyte disengager between the level of foaming and the recirculation rate of the make-up water and electrolyte. By controlling the rate of flow of the electrolyte during operation, optimum efficiency of the cell can be obtained.
Under certain conditions it is desirable to be able to vary the electrolyte flow rate between the anolyte disengager and the anodes to control the level of foam build-up within the disengager. During the start-up of the cell, a period which can last from initial start-up to 12 hours, the amount of electrolyte being recirculated needs to be limited because of the high level of foaming that occurs in the disengager. Gradually, as the cell stabilizes, the electrolyte flow rate could possibly be increased. Also, variations in the current level which the cell receives during operation in response to increased or decreased production demands for caustic or chlorine, or power outages can require a change in the electrolyte flow rate during recirculation to maintain the foam build-up and chlorine gas separation at the optimum levels in the anolyte disengager. Varying levels of carbonate in the feed brine that is used as the electrolyte can substantially affect the amount of foam that is produced in the anolyte disengager. This occurs because the process generates CO.sub.2 gas which bubbles up through electrodes with the other gases which are produced to contribute to the foam layer in the disengager. Further, any attempt to optimize the disengaging rate of the gas in the anolyte disengager from the anolyte fluid can require variation in the flow rate of the recirculating electrolyte fluid during operation.
Higher than normal levels of foaming can occur in the catholyte disengager during the start-up of a cell lasting from initial start-up for as long as 4 to 6 hours. Similarly to the anolyte disengager, as the cell gradually stabilizes, the electrolyte and make-up water recirculation rate could possibly be increased to optimize the rate of gas separation within the catholyte disengager.
The size of the anolyte and catholyte disengagers are a direct function of the foaming levels and amount of gas separation desired within each disengager. Where excessive foaming continually occurs, larger sized disengagers may be required. An alternative approach providing satisfactory performance can be achieved by varying the electrolyte flow rate through the cell. In fact, it is entirely possible that by varying the flow rate, smaller sized disengagers could be utilized. This is especially attractive for anolyte disengagers where the construction involves costly materials, such as titanium.
The foregoing problems are solved in the design of the apparatus comprising the present invention by providing a variable flow restrictor in the flow conduit from each gas-liquid disengager to each electrode frame to selectively vary the flow rate of the electrolyte fluid being recycled through the disengagers to each electrode to thereby control the level of foaming in the electrolyte fluid within the disengagers and thereby optimize the amount of gas separated out within the disengagers.